1. Field of the Invention
The present invention relates to a toner for developing an electrosatic latent image formed on an electrostatic latent image-supporting member.
2. Description of the Prior Art
An image forming method which includes the steps of developing an electrostatic latent image formed on an electrostatic latent image-supporting member, such as potosensitive member, by using a toner, and transferring the resulting toner image onto a recording medium, such as recording paper, has been widely employed in copying machines, printers, facsimile, and also in full-color image forming apparatus which reproduces a multicolor image by superposing a plurality of color toners one over another.
Such an electrostatic latent image-developing toner to be used in various types of image forming apparatuses is required to have different characteristics according to the types of such apparatus. In image forming apparatuses of a digital system, for example, a variable area gradation system or a laser intensity modulation system is adopted as a variable contrast image reproduction system. Whichever system of the two may be employed, however, for good image tone reproduction, the toner is required to have high fluidity. Especially where the laser intensity modulation system is employed, higher fluidity is required of the toner so as to enable tone reproduction to conform to any toner deposit variation corresponding to a change in the charge quantity of the latent image due to laser intensity modulation. Further, for use in a full-color image forming apparatus, wherein toners of different colors are subjected to multiple transfer for full-color image reproduction, the toner must have good transfer characteristics.
Since full color toners are such that color reproduction is carried out by a mixture of different color toners, for such color reproduction it is necessary that the full color toner must have good light transmission characteristics. Therefore, the binder resin used in the toner particles must possess sharp melt characteristics. Unfortunately, however, a toner having such characteristics is apt to cause toner aggregation due to a stress or the like imposed upon the toner in the developing apparatus during voluminous printing. This poses the possibility of void occurrence in a solid image that is attributable to the toner aggregation.
Further, full color toners are required to achieve higher grade halftone reproduction and finer particle feature than in the case of monochrome toners, and this necessitates particle size reduction with respect to full color toners. However, particle size reduction may tend to cause the toner to be adversely affected by heat and/or stress, resulting in toner particle agglomeration, and also in fluidity and cleanability degradation.
In order to meet the foregoing characteristic requirements, therefore, various problems exist to be overcome from technical standpoints. Conceivably, for example, one effective approach for fluidity improvement is to externally add a fluidizing agent, such as fine silica particulate or fine titania particulate, to the toner, thereby to increase the quantity of addition of such an agent. However, increased addition of external additives tends to result in an increase in the quantity of the external agent added to the toner which may pass through the cleaning blade and adhere to the surface of the potosensitive member. This in turn poses a problem such that the external agent which has adhered to the potosensitive member surface acts as a nucleus to which other toner components may adhere in a trailing fashion during the process of cleaning. As a result, the problem of external agent retention on the potosensitive member surface (hereinafter referred to as "BS") is pronounced. If the quantity of the external additive is reduced with a view to preventing the occurrence of BS, the fluidity of the toner becomes insufficient; in addition, toner aggregation may occur due to a stress or the like exerted on the toner in the developing apparatus during voluminous printing, and this may lead to the problem of voids in a solid image. A toner of reduced particle size naturally has an increased fine powder content, so that aforementioned problem is more pronounced with respect to the toner. If a spherical toner of reduced particle size is used for improvement of image quality and transfer performance, toner particles passing through the cleaning blade will increase in number, thus causing unsatisfactory cleaning which will in turn lead to image noise generation.